Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01Q—ANTENNAS, i.e. RADIO AERIALS
  • H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
  • H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
  • H01Q3/267—Phased-array testing or checking devices

Definitions

• Phased array antenna having integral calibration network and method for measuring calibration ratio thereof
• This invention relates to phased array antennas and in particular to calibration of phased array antennas having field calibration capability.
• a phased array antenna comprises a number transmitting/receiving elements, usually arranged in a planar configuration. Each element, or a group of elements, is driven by a transmit/- receive (T/R) module which controls the phase and the amplitude of the corresponding antenna element.
• T/R transmit/- receive
• the signal On transmission of a signal from a phased array antenna, the signal is divided into a number of sub-signals, and each sub-signal is fed to one of the modules.
• the modules comprise signal channels guiding the sub-signals to the antenna elements.
• Each signal channel comprises controllable attenuators or amplifiers and controllable phase- shifting devices for controlling the amplification and the phase shift of the modules.
• the signals transmitted through the antenna elements interfere with each other. By selecting suitable values of the relative amplification and the relative phase-shifting between the modules and by utilizing the interference of the transmitted signals, the directional sensitivity of the antenna can be controlled.
• the opposite procedure takes place compared to transmission. Each antenna element receives a sub-signal.
• the modules comprise signal channels for reception and through these signal channels the sub-signals are collected in a single point in which all sub-signals are added to form a single composite signal.
• the signal channels for reception also comprise amplifiers and phase shifters, and the directional sensitivity of the antenna for reception can be controlled in a corresponding way as for transmission, by varying the amplification and phase-shifting of the modules.
• phase shift signal that is fed to the second antenna element must have a phase offset of -15° relative to the phase shift signal fed to the first antenna in order to compensate for the mismatch in the two phase shifters.
• Differences between the amplitudes of signals that are output by different antenna elements caused by mismatches in the gains of the amplifiers coupled to the antenna elements are compensated for in a similar manner by applying different gain offsets to the antenna elements relative to a given reference antenna element.
• Phased array antenna architectures typically include a calibration network, whose purpose is to provide injection of a predetermined calibration signal to each antenna element and to the T/R module connected to it.
• a calibration network is shown in US Patent 7,068,218 (Gottl et al.) which describes a calibration device for an antenna array, or an improved antenna array, that can be viewed as a set of RF-couplers (one coupler per antenna element) interconnected and driven by a passive network having a common feed point.
• the passive network splits the drive signal in a predetermined manner so that the signal fed to each antenna element is known in advance and the phase and gain offsets are known and predetermined.
• one or more antenna elements may become out of calibration. This can occur, for example, owing to one or more antenna elements being replaced. Since the replacement antenna elements will inevitably have slightly different properties to the original antenna elements, the original offsets will not compensate for slight differences in the phase and gain characteristics of the phase shifters and amplifiers used to feed steering signals to the replacement antenna elements. This typically requires that the complete phase antenna array be returned to the factory for re-calibration in order to establish the new offsets. It is also known to perform the re-calibration procedure in the field, but this then requires a calibration network for which the required offsets are known for each phase shifter and amplifier. Such calibration networks are available but they require sophisticated electronics and are expensive.
• US Pat. No. 7,068,218 G ⁇ ttl et al. discloses a calibration procedure that utilizes, in addition to the operational transmit/receive channels, also an auxiliary injection network, whose contribution must be known in advance. This is determined using the concept of the calibration ratio, which measures the ratio between signals injected externally (in principle from infinity) to those injected internally.
• Some antennas are factory calibrated. When deployed, the quality of the calibration is tested by one means or another and if the test fails the antenna is sent back to the factory for recalibration. Other antennas have field calibration capability. A number of approaches for calibration of such antennas have been proposed in prior art.
• the other approach disposes the external calibration source proximate each antenna element in turn, while ensuring that the distance from the external calibration source to each antenna element is the same and that the external calibration source is exactly aligned to the optical center of each antenna element. This also ensures that the respective amplitudes and phases of the external calibration signals injected into each antenna element are the same, but requires critical and consequently complex alignment and is both time-consuming and expensive.
• T/R module Replacement of a failed T/R module during antenna maintenance is a routine procedure, which requires recalibration of the antenna system.
• the amplification and phase shift of the T/R modules are obtained by considering the change in amplitude and phase of the test signal when it passes the T/R module.
• the control signals controlling the attenuators and the phase shifters in the T/R modules can now be corrected so that the amplification and the phase-shift are made to coincide with the desired amplification and phase-shift.
• a plane wave RF-source is used to simulate a point RF-source at infinity. If the propagation direction of the plane wave is parallel to the bore sight axis of the plane array, all array antenna elements are in the same phase conditions. This means that ideally measured phase values of the signal received by all array antenna elements are identical since each pair of array antenna elements and T/R module is assumed to be identical.
• the calibration procedure enables amplitude and phase characteristics of each pair of antenna element and T/R module to be determined.
• the calibration system includes a probe located in the near field, and a calibration tone generator.
• the near field calibration procedure can be applied to transmit or receive modes as well, hi case of receive calibration mode, a probe sequentially moves from one antenna element to another, keeping the same coupling conditions (distance from antenna plane, polarization, orientation etc.) and transmitting the same test signal.
• a receive antenna array has a switching arrangement, providing appropriate RF-module/antenna element connection to the measurement unit via controllable phase shifter/attenuator.
• the near- field calibration goal achieves the same signal parameters (phase and amplitude) coming from each RF-module (and appropriate probe locations) by applying control signals to the appropriate phase shifters and attenuators.
• far field calibration allows the calibration signal to be fed simultaneously to all the antenna elements from a common source and ensures that it will arrive at the same phase at all the antenna elements; but is not suitable for use in confined spaces, such as when re-calibrating antenna elements in the field.
• near field calibration requires that in order for the external calibration signal to arrive at the same phase at all the antenna elements, it must be fed to each antenna element sequentially and this requires precise alignment which is time-consuming and expensive.
• a phased antenna arrangement in accordance with an embodiment of the invention comprises an array antenna per se, including a plurality of antenna elements, a plurality of receiving channels, an injection unit for injection of calibrating signals into the receiving channels, a point RF-source, located in a far field zone, a distance measurement unit, an amplitude and phase measurement unit and a data processing unit.
• a method for estimating the calibration ratio of an active phased antenna having a plurality of phased array antenna elements comprising: injecting an internal calibrating signal having a known amplitude and phase to each antenna element; sequentially injecting an external calibration signal from a stationary RF-source to all of the phased array antenna elements so that different phases of the external calibration signal arrive at each of the antenna elements; compensating for differences in phases of the external calibration signal reaching the antenna elements so as compute an effective signal amplitude that would reach all of the antenna elements at zero phase difference; calculating calibration ratio as the ratio between the amplitude of the internal calibrating signal to the effective signal amplitude of the external calibration signal; and outputting said calibration ratios in a form for allowing calibration of the active phased antenna.
• a calibration ratio calculation system for use in calibrating a phased array antenna arrangement having a first plurality of phased array antenna elements connected to a second plurality of receiving channels, an integral calibration signal injection network for injecting respective calibration signals to each antenna element and an amplitude and phase measurement unit for measuring respective signal amplitude and phase for each antenna element
• the calibration ratio calculation system comprising: a probe for disposing in the near field of an aperture of the phased array antenna arrangement for injecting an external calibration signal from a stationary RP-source to all of the phased array antenna elements via a respective receiver connected to each of the antenna elements so that different phases of the external calibration signal arrive at each of the antenna elements, a signal correction unit for computing and applying a respective phase difference and amplitude difference to the respective external calibration signal for each antenna element so as to obtain a corrected external calibration signal at all of the antenna elements whose phase difference and amplitude difference is zero; and a calibration ratio processing unit coupled to the signal correction unit for calculating a complex number calibration ratio as the ampli
• a calibration system for calibrating a phased array antenna arrangement having a first plurality of phased array antenna elements connected to a second plurality of receiving channels, an integral calibration signal injection network for injecting respective calibration signals to each antenna element and an amplitude and phase measurement unit for measuring respective signal amplitude and phase for each antenna element
• said calibration system comprising: a probe disposed in the near field of an aperture of the phased array antenna arrangement for injecting an external calibration signal from a stationary RF-source to all of the phased array antenna elements via a respective receiver connected to each of the antenna elements so that different phases of the external calibration signal arrive at each of the antenna elements, a signal correction unit for computing and applying a respective phase difference and amplitude difference to the respective external calibration signal for each antenna element so as to obtain a corrected external calibration signal at all of the antenna elements whose phase difference and amplitude difference is zero; and a calibration ratio processing unit coupled to the signal correction unit for calculating a complex number calibration ratio as the amplitude ratio and the phase difference of
• a calibration signal injection network for injecting respective calibration signals to each antenna element of a phased array antenna arrangement having an amplitude and phase measurement unit for measuring respective signal amplitude and phase for each antenna element
• said calibration signal injection network comprising: a corporate feed for injecting an internal calibration signal to said antenna elements; a plurality of signal dividers connected to the corporate feed; and a plurality of couplers connected to the dividers for conveying a fraction of the internal calibration signal to respective antenna elements of the phased array antenna arrangement; whereby a calibration ratio of the phased array antenna arrangement may be determined regardless of physical changes with time of components and interconnections of the calibration signal injection network by: injecting an internal calibrating signal to the corporate feed; sequentially injecting an external calibration signal from a stationary RF-source to all of the phased array antenna elements so that different phases of the external calibration signal arrive at each of the antenna elements; compensating for differences in phases of the external calibration signal reaching the antenna elements so as compute an effective signal amplitude that would reach all
• Such a calibration signal injection network may be provided integral with a phased array antenna, resulting in a cost-effective phased array antenna arrangement that is amenable to field calibration without expensive and complex alignment procedures.
• a steering/tracking signal is fed to the antenna elements and generates a charge/current distribution over the antenna aperture corresponding to a desired far field antenna pattern.
• This distribution is governed by certain controls applied to Tx/Rx modules in the corresponding receiving channels which are separated from the antenna aperture by cables and other electrical components. The determination of these controls is affected by the cables and components and by the desired current distribution.
• the calibration procedure to which the present invention is directed serves to estimate the contribution of cables and other electric components. This procedure must be repeated quite often, especially when the ambient temperature changes significantly.
• the electrical paths, over which signals flow during actual use of the phased array antenna and during calibration, are not identical. That is to say there is a different path that is used for operational purposes to the one used for maintenance purposes — calibration being one of them.
• the signals used in the calibration procedure flow through the channel which is calibrated and also through the internal injection network, which constitutes the difference between the two paths.
• the gateway between the channel and the internal injection network is implemented by a plurality of couplers located in the antenna in one-to-one correspondence with antenna elements. Since signals used in operational modes come and go from/to infinity while those in calibration come and go from/to the internal calibration network, the difference among various paths must be compensated for.
• the invention employs a horn since it is easily implemented under field conditions.
• such a method comprises the following process stages: measuring distance between the phased array antenna and the point RF- source, measuring antenna allocation parameters, measuring the signals injected by means for internal injecting calibrating signals and the point RF-source, estimating configuration of phase front emanated by the point RF-source and phase component of calibration ratio using regression analysis.
• prior art calibration methods enable each pair of array antenna elements and transmitting/receiving channels to be calibrated together only. Replacement of an array antenna element and/or one/or several transmitting/ receiving channels results in a lack of calibration.
• plane wave RF-sources are extremely expensive and unwieldy, recalibration in the field conditions is time- consuming and expensive.
• Fig. 1 shows a simple calibration signal injection network that may be used with the invention
• Fig. 2 is a block diagram of the phased array antenna arrangement using the point RF-source for calculating calibration ratio according to an embodiment of the invention
• Fig. 3 is a pictorial representation showing the spatial arrangement of the point RF-source and a plurality of phased array antenna elements according to an embodiment of the invention
• Fig. 4 is a block diagram showing the functionality of a system for calculating calibration ratio according to an embodiment of the invention.
• Fig. 5 is a flow diagram showing a sequence of operations for calculating calibration ratio according to an embodiment of the invention.
• Fig. 1 shows a simple calibration signal injection network 10 having a triad of dividers 11, 12 and 13 interconnected so that a common junction of the dividers 11 and 12 serves as a corporate feed point 14 for injecting an input signal into the network.
• Respective junctions between opposite ends of the divider 13 and respective ends of the dividers 1 1 and 12 are connected to similar divider triads comprising dividers 15, 16, 17 and 18, 19, 20.
• the dividers 15 and 16 are commonly connected at a first end to one end of the divider 13 whose other end is commonly connected to a first end of the dividers 18 and 19.
• the second ends of the dividers 15, 16, 18 and 19 are connected to respective couplers 21 each of which is terminated by a respective termination 26.
• the input signal is split initially at the junction between the dividers 11 and 12 and is again split at each of the respective junctions between dividers 15, 16 and 18, 19. Depending on the values of the dividers, different currents will flow through each of the couplers 21. Referring to Fig. 1 and Fig. 3 together, the calibration signal injection network
• the 10 is interposed between an array of antenna elements 31 and a ground plane 25, so that when a single input signal is fed to the corporate feed point 14 of the calibration network 10, respective steering signals are fed to each of the antenna elements 31 via respective phase shifters and amplifiers that are known per se and are not shown in the figures and that can be inductively coupled to the couplers 21.
• the values of the steering signals fed to each antenna element are predetermined by the values of the dividers in the calibration network 10 and are thus known in advance.
• an antenna array is calibrated using the calibration signal injection network 10
• an input signal is fed to the corporate feed point 14 and the output signals flowing through each antenna element is measured. Any offset in amplitude or phase from a respective desired value is measured and the corresponding amplitude and phase offsets are determined.
• Fig. 2 shows a phase array antenna arrangement 30 that includes a plurality of array antenna elements 31 , a ground plane (not shown), a plurality of receiving channels 32, an internal injection unit 33 for injecting calibrating signals, a point RF-source 35, an amplitude and phase measurement unit 36, a distance measurement unit 37 and a processing unit 38 having a memory 39.
• Each antenna element 31 is connected to a respective receiving channel 32. Signals received by the receiving channels 32 are measured by the amplitude and phase measurement unit 36 and the measured data are stored in the memory 39 and processed by the data processing unit 38.
• the internal injection unit 33 that is coupled to antenna elements 31 and to the receiving channels 32, while the second is the point RF-source 35 from which a spherical wave 40 emanates toward the plurality of the antenna elements 31.
• the second is the point RF-source 35 from which a spherical wave 40 emanates toward the plurality of the antenna elements 31.
• Comparison of measurement results of these two signals enables derivation of the so-called phase component of calibration ratio attributed to the plurality of antenna elements.
• Statistical methods of data processing used in this invention enable the estimation accuracy to be improved owing to repeated measurements that are performed at slightly different geometrical conditions. Signals provided by the injection unit 33 are considered stable and are measured only once per session.
• Fig. 3 shows the spatial arrangement of the point RF-source 35 and the plurality of array antenna elements 31 in the coordinate system.
• phase front having a smooth and continuous spherical surface corresponding to a geometrical location of points that are equidistantly located relative to the phase centre of the source.
• This phase front can be viewed as spherical, if it is in the far field zones during each independent measurement.
• D ⁇ maximum aperture dimension
• r is the distance from the phase centre of the RF-source
• D is an aperture of the RF-source
• ⁇ is the wavelength of the RF-radiation
• a real point RF-source with aperture A ⁇ may be placed at a distance of 50 ⁇ or greater.
• the signal emanated by the point RF-source 35 and measured by the amplitude and phase measurement unit 36 is subject to phase delay at several points: (i) transfer of the spherical wave 40 from the point RF-source 35 to the antenna elements 31; (ii) "phase shift" at the antenna elements 31; (iii) phase change in the receiving channels 32.
• the signal injected by the injection unit 33 into the receiving channels 32 is subjected to the phase change caused by passing through the plurality of receiving channels 32, i.e.
• ⁇ PS ( P ⁇ + ⁇ CR + ⁇ (2) where: ⁇ ps is the measured phase value of the point RF-source 35,
• ⁇ p ⁇ is the phase shift caused by wave transfer from the point RF-source 35 to the antenna elements 31,
• ⁇ CR is the phase shift on the antenna elements 31, and ⁇ is the phase value of the internal calibrating signal.
• Transfer of the spherical wave front 40 from the point RF-source 35 to the j-th antenna element 31 produces a phase difference given by: where X 3 , Y ]t 2, are the coordinates of the j-th antenna element 31, and Xps, Yps, Zps are coordinates of the point RF-source 35.
• phase difference is given by:
• ⁇ ⁇ (R,j) ⁇ yX J 2 +R 2 +Z; -RJ (4)
• the antenna element lattice is rectangular with element separation about ⁇ /2.
• the peripheral elements can have wave front phases different from that of the central element by approximately 8 ⁇ , but the phase difference between neighboring elements does not exceed 0.18 ⁇ .
• the fact that the phase difference between neighboring elements is only a small fraction of the complete cycle allows for an unwrapping algorithm to resolve the intrinsic ambiguity caused by arithmetic operations on periodic operands i.e. phases.
• ⁇ CR U) ⁇ PS U) ⁇ ⁇ l U) ⁇ ⁇ l U) (5)
• the method of calibration ratio estimation includes two stages: performing measurements and data processing.
• Fig. 4 is a block diagram showing the functionality of a calibration ratio calculation system 45 for use in calibrating a phased array antenna arrangement 30 such as shown in Fig. 1.
• the calibration ratio calculation system 45 comprises a probe 46 for disposing in the near field of an aperture of the phased array antenna arrangement for injecting an external calibration signal from a stationary RF-source to all of the phased array antenna elements via a respective receiver connected to each of the antenna elements so that different phases of the external calibration signal arrive at each of the antenna elements.
• the calibration ratio calculation system 45 further comprises a signal correction unit 47 for computing and applying a respective phase difference and amplitude difference to the respective external calibration signal for each antenna element so as to obtain a corrected external calibration signal at all of the antenna elements whose phase difference and amplitude difference is zero.
• a calibration ratio processing unit 48 is coupled to the signal correction unit 47 for calculating a complex number calibration ratio as the amplitude ratio and the phase difference of the internal calibrating signal relative to the corrected external calibration signal.
• Fig. 5 is a flowchart showing the principal operations required to estimate calibration ratio according to an embodiment of the invention.
• the sequence of operations includes the following: a. inject internal calibrating signal to all antenna elements, b. measure and store the injected signal (the signal is sampled and digitized by the receiving channel (32 in Fig. 2) and the amplitude and phase are measured by the amplitude and phase measurement unit (36 in Fig. 2)) c. Start the following loop for successive iterations: i) place a point RF-source in the working position, ii) measure the distance between the point RF-source (35 in Fig. 2) and phase center of the antenna elements (31 in Fig.
• iii) optionally store measurement data for subsequent retrieval by a different unit, although this is not necessary if subsequent processing is carried out either by the same unit or by one coupled thereto, iv) load stored data if subsequent processing is carried out by a different unit, v) calculate an approximate value of ⁇ c R of wave front, vi) injecting external calibrating signal using the point RF-source at the current working position, vii) measuring the RF signal from external source, viii) storing measurement results, ix) calculate approximate value of calibration ratio, x) place a point RF-source in another working position, xi) inject external calibrating signal using the point RF-source at the new working position, xii) measure and store the signal from external source at the new working position, xiii) optionally store measurement data for subsequent retrieval by a different unit, although this is not necessary if subsequent processing is carried out either by the same unit or by one coupled thereto, xiv) load stored measurement data if subsequent processing is carried out by a
• xvi) calculate an updated value of the phase component of the calibration ratio for the point RF-source in the new position
• xvii) calculate error as weighted difference between two sets of calibration ratios
• xviii) if error is not less than specified threshold, perform successive iteration
• the calibration ratios are tabulated and used to apply corrections to the amplitude and phase of the fractional external calibration signal applied to each antenna element as explained above.
• Fig. 2 and Fig. 5 together, for the sake of clarity we will limit our consideration to two working positions (i.e. just two iterations) of the point RF-source, but note that the algorithm may be repeated using different positions so as to smooth out noisy measurements.
• the internal calibration is implemented.
• the injection unit 33 is assumed to be stable, therefore ⁇ / is measured only once for each session.
• the injection unit 33 injects the signal into each receiving channel 32.
• Each signal passing through the receiving channel 32 is measured by the amplitude and phase measurement unit 36. Measurement data are stored in the memory 39. To calculate the phase shift at each array antenna element, the location of the antenna element must be known. Therefore the parameters of the array antenna element allocation are measured and stored.
• the first cycle of the procedure starts from placing the point RF-source 35 into a working position.
• a horn antenna used as the point RF-source 35 is placed in proximity of the bore sight axis of the array antenna elements 31 (that coincides with the Y axis in Fig. 2).
• the distance between the point RF-source 35 and the array antenna elements 31 is measured by the distance measuring unit 37, which may be, for example, a laser rangefinder. Measurement data are stored in data processing unit 38.
• At least two measurements of the signal from point RF-source 35 are performed at different locations of the point RF-source 35 relative to the plurality of antenna elements 31.
• ⁇ T ( R2 > J) ⁇ PPS ( R2 > J) ⁇ ⁇ , U) - ⁇ CR (*1, J) (7)
• phase front at this stage of the algorithm is very close to spherical, but this sphere can be rotated, because of displacement of the point RF-source 35 relative to antenna broadside axis.
• Regression analysis methods are applied to the phase front, the steering phase offset ⁇ Trend (R2,j) is calculated and a new (corrected) phase front of point RF-source 35 is updated according to:
• phase front configuration is calculated using regression analysis:
• ⁇ ⁇ (R2J) ⁇ l s (R2J) - ⁇ i (j) - ⁇ CR (R ⁇ J) (9)
• phase centre location of the point RF-source 35 Xps, Y ps, Zps are estimated.
• the phase of wave front for second test point is calculated:
• ⁇ ⁇ (R2,j) 2 jy(X J -X PS (R2)f + ⁇ Y PS (R2)) 2 + ⁇ Z J -Z Ps (R2)f - JTC] (11)
• ⁇ CR (R2J) ⁇ PS (R2J) - ⁇ i (j) - ⁇ ⁇ (R2J) (12)
• ⁇ cn(R2j) ⁇ PS (R2J) - ⁇ i (j) - ⁇ ⁇ (R2J)
• ⁇ CR (RU J) ⁇ PPS ( R l J) ⁇ ⁇ , U) ⁇ ⁇ P ⁇ C ⁇ J> ( X PSI > Y PSI » Z PS ⁇ )) (15)
• This algorithm can be implemented repeatedly or may be terminated.
• the value ⁇ CR obtained in the previous cycle is used for calculating ⁇ rrend in the next cycle.
• system may use a suitably programmed computer or a computer program readable by a computer for executing the method of the invention.
• the invention further contemplates a machine- readable memory tangibly embodying a program of instructions executable by the machine for executing the method of the invention.

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